Which of the following is an IP address that is private (i.e. reserved for internal networks, and not a valid
address to use on the Internet)?

A.
10.0.42.5

B.
11.0.42.5

C.
12.0.42.5

D.
13.0.42.5

Explanation:
The IP address 10.0.42.5 is in the private Class A IP address range.
The private IP address ranges are:
10.0.0.0–10.255.255.255 (Class A network)
172.16.0.0–172.31.255.255 (Class B networks)
192.168.0.0–192.168.255.255 (Class C networks)
Incorrect Answers:
B: 11.0.42.5 is not a private IP address. The first octet must be 10 (or 172, or 192), not 11.
C: 12.0.42.5 is not a private IP address. The first octet must be 10 (or 172, or 192), not 12.
D: 13.0.42.5 is not a private IP address. The first octet must be 10 (or 172, or 192), not 13.

Harris, Shon, All In One CISSP Exam Guide, 6th Edition, McGraw-Hill, New York, 2013, p. 605